Structural Characterization of Metal Hydrides for Energy Applications

Hydrogen can be an unlimited source of clean energy for future because of its very high energy density compared to the conventional fuels like gasoline. An efficient and safer way of storing hydrogen is in metals and alloys as hydrides. Light metal hydrides, alanates and borohydrides have very good hydrogen storage capacity, but high operation temperatures hinder their application. Improvement of thermodynamic properties of these hydrides is important for their commercial use as a source of energy. Application of pressure on materials can have influence on their properties favoring hydrogen storage. Hydrogen desorption in many complex hydrides occurs above the transition temperature. Therefore, it is important to study the physical properties of the hydride compounds at ambient and high pressure and/or high temperature conditions, which can assist in the design of suitable storage materials with desired thermodynamic properties. ^ The high pressure-temperature phase diagram, thermal expansion and compressibility have only been evaluated for a limited number of hydrides so far. This situation serves as a main motivation for studying such properties of a number of technologically important hydrides. Focus of this dissertation was on X-ray diffraction and Raman spectroscopy studies of Mg2FeH6, Ca(BH4) 2, Mg(BH4)2, NaBH4, NaAlH4, LiAlH4, LiNH2BH3 and mixture of MgH 2 with AlH3 or Si, at different conditions of pressure and temperature, to obtain their bulk modulus and thermal expansion coefficient. These data are potential source of information regarding inter-atomic forces and also serve as a basis for developing theoretical models. Some high pressure phases were identified for the complex hydrides in this study which may have better hydrogen storage properties than the ambient phase. The results showed that the highly compressible B-H or Al-H bonds and the associated bond disordering under pressure is responsible for phase transitions observed in brorohydrides or alanates. Complex hydrides exhibited very high compressibility suggesting possibility to destabilize them with pressure. With high capacity and favorable thermodynamics, complex hydrides are suitable for reversible storage. Further studies are required to overcome the kinetic barriers in complex hydrides by catalytic addition. A comparative study of the hydride properties with that of the constituting metal, and their inter relationships were carried out with many interesting features.^

Material Synthesis and Characterization on Low-Dimensional Cobaltates

In this thesis, results of the investigation of a new low-dimensional cobaltates Ba2-xSrxCoO 4 are presented. The synthesis of both polycrystalline and single crystalline compounds using the methods of conventional solid state chemical reaction and floating-zone optical furnace is first introduced. Besides making polycrystalline powders, we successfully, for the first time, synthesized large single crystals of Ba2CoO4. Single crystals were also obtained for Sr doped Ba2-xSrxCoO 4. Powder and single crystal x-ray diffraction results indicate that pure Ba2CoO4 has a monoclinic structure at room temperature. With Sr doping, the lattice structure changes to orthorhombic when x ≥ 0.5 and to tetragonal when x = 2.0. In addition, Ba2CoO4 and Sr2CoO4, have completely different basic building blocks in the structure. One is CoO4 tetrahedron and the later is CoO6 octahedron, respectively. Electronic and magnetic properties were characterized and discussed. The magnetic susceptibility, specific heat and thermal conductivity show that Ba2CoO4 has an antiferromagnetic (AF) ground state with an AF ordering temperature TN = 25 K. However, the magnitude of the Néel temperature TN is significantly lower than the Curie-Weiss temperature (:&thetas;: ∼ 110 K), suggesting either reduced-dimensional magnetic interactions and/or the existence of magnetic frustration. The AF interaction persists in all the samples with different doping concentrations. The Néel temperature doesn't vary much in the monoclinic structure regime but decreases when the system enters orthorhombic. Magnetically, Ba2CoO4 has an AF insulating ground state while Sr2CoO4 has a ferromagnetic (FM) metallic ground state. Neutron powder refinement results indicate a magnetic structure with the spin mostly aligned along the a-axis. The result from a μ-spin rotation/relaxation (μ+SR) experiment agrees with our refinement. It confirms the AF order in the ab -plane. We also studied the spin dynamics and its anisotropy in the AF phase. The results from inelastic neutron scattering show that spin waves have a clear dispersion along a-axis but not along c-axis, indicating spin anisotropy. This work finds the strong spin-lattice coupling in this novel complex material. The interplay between the two degrees of freedom results an interesting phase diagram. Further research is needed when large single crystal samples are available.

Plasma And Cold Sprayed Aluminum Carbon Nanotube Composites: Quantification Of Nanotube Distribution And Multi-Scale Mechanical Properties

Carbon nanotubes (CNT) could serve as potential reinforcement for metal matrix composites for improved mechanical properties. However dispersion of carbon nanotubes (CNT) in the matrix has been a longstanding problem, since they tend to form clusters to minimize their surface area. The aim of this study was to use plasma and cold spraying techniques to synthesize CNT reinforced aluminum composite with improved dispersion and to quantify the degree of CNT dispersion as it influences the mechanical properties. Novel method of spray drying was used to disperse CNTs in Al-12 wt.% Si pre-alloyed powder, which was used as feedstock for plasma and cold spraying. A new method for quantification of CNT distribution was developed. Two parameters for CNT dispersion quantification, namely Dispersion parameter (DP) and Clustering Parameter (CP) have been proposed based on the image analysis and distance between the centers of CNTs. Nanomechanical properties were correlated with the dispersion of CNTs in the microstructure. Coating microstructure evolution has been discussed in terms of splat formation, deformation and damage of CNTs and CNT/matrix interface. Effect of Si and CNT content on the reaction at CNT/matrix interface was thermodynamically and kinetically studied. A pseudo phase diagram was computed which predicts the interfacial carbide for reaction between CNT and Al-Si alloy at processing temperature. Kinetic aspects showed that Al4C3 forms with Al-12 wt.% Si alloy while SiC forms with Al-23wt.% Si alloy. Mechanical properties at nano, micro and macro-scale were evaluated using nanoindentation and nanoscratch, microindentation and bulk tensile testing respectively. Nano and micro-scale mechanical properties (elastic modulus, hardness and yield strength) displayed improvement whereas macro-scale mechanical properties were poor. The inversion of the mechanical properties at different scale length was attributed to the porosity, CNT clustering, CNT-splat adhesion and Al4C3 formation at the CNT/matrix interface. The Dispersion parameter (DP) was more sensitive than Clustering parameter (CP) in measuring degree of CNT distribution in the matrix.

Desenvolvimento biotecnológico de uma emulsão de uso tópico a base de óleo de rã-touro Rana catesbeiana Shaw

The skin is one of the largest organs of the human body and accounts for about 16% of body weight. The body protection against the external environment microorganisms is one of its most important functions, however is necessary that the skin remain intact for this function be exercised, so that when there is an injury on the skin, the process of restructuring needs to be starts, however this restructuration may also be compromised due to some diseases, justifying even more the need for the development of topical products that promote or accelerate the skin healing. Thus the aim of this study was to extract bullfrog oil and to develop a suitable topical emulsion. Two different oil samples were extracted by hot or organic solvent process. Titration techniques and gas chromatography- mass spectrometry were used to characterize the bullfrog oil. The required hydrophile-lipophile balance (HLBr) of bullfrog oil was determined and a pseudo-ternary phase diagram was constructed. The stability of the topical emulsion was evaluated. Then, cellular viability was determined by MTT assay using normal fibroblasts (3T3) and melanoma (B16F10) cells lines. The hot extraction yield was 60.6%. The major polyunsaturated compounds found were Eicosapentaenoic acid (17.6%) and Arachidonic acid (8.4%). HLBr study demonstrated the presence of stable systems with HLB ranging from 12.1 to 13.5 and the pseudo-ternary phase diagram showed mainly emulsion systems (62%). Topical emulsion showed 390 nm, polydispersity 0.05, zeta potential -25 mV and remained stable for ninety days. The bullfrog oil and topical emulsion did not showed citotoxicity in normal fibroblasts cells. However, these systems showed significantly inhibition of melanoma cells growth. In conclusion, the bullfrog oil presented desirable chemical characteristics required to be used for the development of a pharmaceutical and cosmetic products.

Numerical study of the enlarged O(5) symmetry of the 3D antiferromagnetic RP^(2) spin model

We investigate by means of Monte Carlo simulation and finite-size scaling analysis the critical properties of the three dimensional O (5) non-linear σ model and of the antiferromagnetic RP^(2) model, both of them regularized on a lattice. High accuracy estimates are obtained for the critical exponents, universal dimensionless quantities and critical couplings. It is concluded that both models belong to the same universality class, provided that rather non-standard identifications are made for the momentum-space propagator of the RP^(2) model. We have also investigated the phase diagram of the RP^(2) model extended by a second-neighbor interaction. A rich phase diagram is found, where most of the phase transitions are of the first order.

Dynamical generation of a gauge symmetry in the double-exchange model

It is shown that a bosonic formulation of the double-exchange model, one of the classical models for magnetism, generates dynamically a gauge-invariant phase in a finite region of the phase diagram. We use analytical methods, Monte Carlo simulations and finite-size scaling analysis. We study the transition line between that region and the paramagnetic phase. The numerical results show that this transition line belongs to the universality class of the antiferromagnetic RP^(2) model. The fact that one can define a universality class for the antiferromagnetic RP^(2) model, different from the one of the O(N) models, is puzzling and somehow contradicts naive expectations about universality.

Fast pyrolysis oil fuel blend for marine vessels

The main driver for the investigation of fast pyrolysis oil marine fuel blends is EU directive 2012/33/EU which aims to cut the sulphur content of marine fuel and thereby reduce air pollution caused by marine vessels. The aim of this study was to investigate the miscibility of 3- and 4- component blends containing pyrolysis oil, 1-butanol, biodiesel (RME) and/or marine gas oil (MGO). The ideal blend would be a stable homogenous product with a minimum amount of butanol, whilst maximising the amount of pyrolysis oil. A successful blend would have properties suitable for use in marine engines. In order to successfully utilise a marine fuel blend in commercial vessels it should meet minimum specification requirements such as a flash point ≥60°C. Blends of pyrolysis oil, RME, MGO and 1-butanol were evaluated and characterised. The mixed blends were inspected after 48 hours for homogeneity and the results plotted on a tri-plot phase diagram. Homogenous samples were tested for water content, pH, acid number, viscosity and flash point as these give indicate a blend’s suitability for engine testing. The work forms part of the ReShip Project which is funded by Norwegian industry partners and the Research Council of Norway (The ENERGIX programme).

Fermion Mass Generation without Spontaneous Symmetry Breaking

The conventional mechanism of fermion mass generation in the Standard Model involves Spontaneous Symmetry Breaking (SSB). In this thesis, we study an alternate mechanism for the generation of fermion masses that does not require SSB, in the context of lattice field theories. Being inherently strongly coupled, this mechanism requires a non-perturbative approach like the lattice approach.

In order to explore this mechanism, we study a simple lattice model with a four-fermion interaction that has massless fermions at weak couplings and massive fermions at strong couplings, but without any spontaneous symmetry breaking. Prior work on this type of mass generation mechanism in 4D, was done long ago using either mean-field theory or Monte-Carlo calculations on small lattices. In this thesis, we have developed a new computational approach that enables us to perform large scale quantum Monte-Carlo calculations to study the phase structure of this theory. In 4D, our results confirm prior results, but differ in some quantitative details of the phase diagram. In contrast, in 3D, we discover a new second order critical point using calculations on lattices up to size $ 60^3$. Such large scale calculations are unprecedented. The presence of the critical point implies the existence of an alternate mechanism of fermion mass generation without any SSB, that could be of interest in continuum quantum field theory.

Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa

Iron is the main constituent of the core of rocky planets; therefore, understanding its phase diagram under extreme conditions is fundamental to model the planets’ evolution. Using dynamic compression by laser-driven shocks, pressure and temperature conditions close to what is found in these cores can be reached. However, it remains unclear whether phase boundaries determined at nanosecond timescales agree with static compression. Here we observed the presence of solid hexagonal close-packed iron at 170 GPa and 4,150 K, in a part of the iron phase diagram, where either a different solid structure or liquid iron has been proposed. This X-ray diffraction experiment confirms that laser compression is suitable for studying iron at conditions of deep planetary interiors difficult to achieve with static compression techniques.

Interplay of Defects, Magnetism, Ripples and Strain in Graphene

We present a comprehensive study based on first-principles calculations about the interplay of four important ingredients on the electronic structure of graphene: defects + magnetism + ripples + strain. So far they have not been taken into account simultaneously in a set of ab initio calculations. Furthermore, we focus on the strain dependence of the properties of carbon monovacancies, with special attention to magnetic spin moments. We demonstrated that such defects show a very rich structural and spin phase-diagram with many spin solutions as function of strain. At zero strain the vacancy shows a spin moment of 1.5 Bohrs that increases up to 2 Bohrs with stretching. Changes are more dramatic under compression: the vacancy becomes non-magnetic under a compression larger than 2%. This transition is linked to the structural modifications associated with the formation of ripples in the graphene layer. Our results suggest that such interplay could have important implications for the design of future spintronics devices based on graphene derivatives, as for example a spin-strain switch based on vacancies.

Aspects of Chiral Symmetry Breaking in Lattice QCD

Thesis (Ph.D.)--University of Washington, 2016-08

(Quantum-) critical dynamics of magnetoelectric materials

In this thesis the critical dynamics of several magnetoelectric compounds at their phase transition were examined. Mostly measurements of the dielectric properties in the frequency range of below 1 Hz up to 5 GHz were employed to evaluate the critical exponents for both magnetic field and temperature-dependent measurements. Most of the materials that are part of this work show anomalous behavior, especially at very low temperatures where quantum fluctuations are of the order of or even dominate those induced thermally. This anomalous behavior manifests in different forms. In Dy2Ti2O7 we demonstrate the existence of electric dipoles on magnetic monopoles. Here the dynamics at the critical endpoint located at 0.36K and in a magnetic field of 1T parallel to the [111] direction are of special interest. At this critical endpoint the expected critical slowing down of the dynamics could not only not be observed but instead the opposite, critical speeding-up by several orders of magnitude, could be demonstrated. Furthermore, we show that the phase diagram of Dy2Ti2O7 in this field direction can be reproduced solely from the dynamical properties, for example the resonance frequency of the observed relaxation that is connected to the monopole movement. Away from this point of the phase diagram the dynamics are slowing-down with reduction of temperature as one would expect. Additional measurements on Y2Ti2O7, a structurally identical but non-magnetic material, show only slowing down with reduction of temperature and no additional features. A possible explanation for the observed critical speeding-up is a coherent movement of magnetic monopoles close to the critical field that increases the resonance frequency by reducing the damping of the process. LiCuVO4 on the other hand behaves normally at its phase transition as long as the temperature is higher than 0.4 K. In this temperature regime the dynamics show critical slowing-down analogous to classical ferroelectric materials. This analogy extends also towards higher frequencies where the permittivity displays a ‘dispersion’ minimum that is temperature-dependent but of the order of 2 GHz. Below 0.4K the observed behavior changes drastically. Here we found no longer relaxational behavior but instead an excitation with very low energy. This low energy excitation was predicted by theory and is caused by nearly gapless soliton excitations within the 1D Cu2+ chains of LiCuVO4. Finally, in TbMnO3 the dynamics of the phase transition into the multiferroic phase was observed at roughly 27 K, a much higher temperature compared to the other materials. Here the expected critical slowing-down was observed, even though in low-frequency measurements this transition into the ferroelectric phase is overshadowed by the so-called c-axis relaxation. Therefore, only frequencies above 1MHz could be used to determine the critical exponents for both temperatureand magnetic-field-dependent measurements. This was done for both the peak frequency as well as the relaxation strength. In TbMnO3 an electromagnetic soft-mode with small optical weight causes the observed fluctuations, similar to the case of multiferroic MnWO4.

Neutron Scattering Studies on Complex Magnetic Structures in Magnetoelectric Materials

In this thesis, the magnetic properties of four transition-metal oxides are presented. Their multiferroic and magnetoelectric phases have been investigated by means of different neutron scattering techniques. The materials TbMnO3 and MnWO4 belong to the group of spin-induced multiferroics. Their ferroelectric polarization can be explained by the inverse DzyaloshinskiiMoriya interaction. Another common feature of both materials is the presence of subsequent magnetic transitions from a spin-density wave to a spin spiral. The features of the phase transitions have been studied in both materials and it could be shown that diffuse magnetic scattering from the spin spiral is present even in the ordered spin-density wave phase. The excitation spectrum in the multiferroic phase of TbMnO3 was investigated in detail and a comprehensive dataset was obtained using time-of-flight spectroscopy. A spin-wave model could be obtained which can quantitatively describe the full dispersion. Furthermore, the polarization of the zone-center excitations could be derived which fit well to data from inelastic neutron spectroscopy and infrared spectroscopy. With the combination of spherical neutron polarimetry and a poling of the sample by an electric field, it was possible to observe the chiral magnetic component of the magnetic excitations in TbMnO3 and MnWO4. The spin-wave model for TbMnO3 obtained in this thesis is able to correctly describe the dispersion of this component. The double tungstate NaFe(WO4)2 is isostructural to the multiferroic MnWO4 and develops a complex magnetic phase diagram. By the use of neutron diffraction techniques, the zero-field structure and high-field structures in magnetic field applied along the b-axis could be determined. The data reveal a direct transition into an incommensurate spin-spiral structure. The value of the incommensurability is driven by anharmonic modulations and shows strong hysteresis effects. The static and dynamic properties in the magnetoelectric spin-glass phase of Ni0.42Mn0.58TiO3 were studied in detail. The spin-glass phase is composed of short-ranged MnTiO3 and NiTiO3-type order. The antiferromagnetic domains could be controlled by crossed magnetic and electric fields, which was visualized using spherical neutron polarimetry. A comprehensive dataset of the magnetic excitations in the spin-glass phase was collected. The dataset revealed correlations in the hexagonal plane which are only weakly coupled along the c-axis. The excitation spectra could be simulated by taking into account the MnTiO3-type order.

Electrolytic cells for plastic waste recycling

The current project assesses potential molten alloy anodes for Solid Oxide Fuel Cells (SOFC) running on solid waste. A detailed phase diagram study was performed to locate probable anode systems. The molten metal oxide system PbO-Sb2O3 was selected as a possible molten alloy anode for this application. A detailed vapour pressure study of this system was performed. Several cells were fabricated to experimentally assess the electrochemical properties of this system. The work reveals several unexpected limiting features such as the incompatibility between the platinum and the chosen alloy. A second cell was built, this time using rhenium wires instead, preventing such reaction. However, the rhenium wire sublimes under oxidizing conditions (air) and the sealing glass and the chosen alloy system react with each other under long term use. Considering all these issues, a third cell design was conceived, surpassing some obstacles and providing some initial information regarding the electrochemical behaviour. The current project shows that many parameters need to be taken into account to ensure materials compatibility. For the PbOSb2O3 system, the high volatility of Sb2O3 was a serious limitation that can only be addressed through the application of new contact wires or sealing materials and conditions. Nonetheless, the project highlights several other potential systems that can be considered, such as Pb11Ge3O17, Pb3GeO5, Pb5Ge3O11, Bi2CuO4, Bi2PdO4, Bi12GeO20.

Bosonization and entanglement spectrum for one-dimensional polar bosons on disordered lattices

Ultra cold polar bosons in a disordered lattice potential, described by the extended Bose-Hubbard model, display a rich phase diagram. In the case of uniform random disorder one finds two insulating quantum phases-the Mott-insulator and the Haldane insulator-in addition to a superfluid and a Bose glass phase. In the case of a quasiperiodic potential, further phases are found, e.g. the incommensurate density wave, adiabatically connected to the Haldane insulator. For the case of weak random disorder we determine the phase boundaries using a perturbative bosonization approach. We then calculate the entanglement spectrum for both types of disorder, showing that it provides a good indication of the various phases.